Forward-declare struct private_handle_t in public header

Headers should include or forward-declare types that they are using.
This breaks a build failure in hardware/qcom/media due to the unknown
type.

Issue: FP2P-432
Change-Id: Ic0ed7957f272fa6983ca0a1d19a31bc2d9eebbfb
diff --git a/libqdutils/qdMetaData.h b/libqdutils/qdMetaData.h
index a73ed70..d7504eb 100644
--- a/libqdutils/qdMetaData.h
+++ b/libqdutils/qdMetaData.h
@@ -111,6 +111,8 @@
     UPDATE_COLOR_SPACE = 0x0400,
 } DispParamType;
 
+struct private_handle_t;
+
 int setMetaData(private_handle_t *handle, DispParamType paramType, void *param);
 
 #endif /* _QDMETADATA_H */